In-app billing promises to be an important source of revenue for developers. On iOS, free apps now make up 14 of the 25 top-grossing apps in the U.S. today, reflecting a strong industry shift away from paid apps. On Android, it may be even more important given the considerable piracy issues developers are facing on the platform.

Overall, Chu said revenue in Android Market is ramping up quickly with a threefold increase in the U.S. over last year. Smaller markets like the Japan have seen a 100-fold increase, while revenue earned through Android Market jumped by eightfold in the U.K.

Direct carrier billing is important to the platform, since Google doesn’t have the more than 200 million iTunes accounts (and credit card details) that Apple has. This creates friction in the payment process since users have to fill out a lengthy form the first time they want to make a purchase in the store.

The number of users either buying apps or making in-app purchases has also jumped by a factor of 6.5 year over year.